Is this a bug in 5.4p1?

[JCA]

   I am testing with a 5.4p1 client and have noticed, on the server
side, that sometimes an SSH_MSG_DISCONNECT message is received with
the following 28-byte long payload:

0x00    0x00    0x00    0x0b
                   Reason: SSH_DISCONNECT_BY_APPLICATION
0x00   0x00     0x00    0x14
                   Description string length: 20 bytes
0x64    0x69    0x73    0x63    0x6f   0x6e   0x6e    0x65
 Description string: disconnected by user
0x63    0x74    0x65    0x64    0x20  0x62   0x79    0x20
0x75    0x73    0x65    0x72

Is this not incomplete? In RFC 4253, section 11.1, the payload for
this message is defined to be

      uint32    reason code
      string    description in ISO-10646 UTF-8 encoding [RFC3629]
      string    language tag [RFC3066]

Is it the case that if the sender does not want to use a language tag
it can omit the language string altogether, or must one still send an
empty string - i.e. a sequence of four bytes all set to zero?